Study On The Effect Of Submersed Buoy On The Taut-slack Characteristics Of The Mooring Line

The new catenary mooring system with additional buoy not only maintains the advantages of catenary mooring system,but also overcomes the disadvantages of low horizontal mooring efficiency and small platform payload caused by excessive weight of mooring line,and expands the application in deep water and ultra-deep water.The additional buoy will certainly affect the taut-slack characteristics and motion response of mooring line,and the alternate process of taut-slack of mooring line may damage the mooring system and affect the stability of the platform.Therefore,it is necessary to study the effect of additional buoy on the taut-slack characteristics of mooring line and its motion response.In this paper,the numerical simulation of the catenary mooring system with additional buoys is carried out.The main work includes the following aspects:(1)The differential equations of motion of the catenary mooring line with buoy were established using the lumped-mass method,the Newton-Raphson method for solving nonlinear equations is introduced,the static configuration of mooring line with buoy in XOZ plane and the tension distribution of each segment are calculated,the influence of the size and position of buoy on the static configuration and tension distribution of mooring line is analyzed.(2)The dynamic tension of the additional buoy mooring line and the motion response of each node were calculated by applying Newton-raphson method combined with Howbolt difference scheme,and the taut-slack characteristics of the mooring system without buoy were analyzed.The results show that when the motion frequency or amplitude of the upper end of the mooring line is large enough to a certain extent,the mooring line will exhibit alternating taut-slack phenomenon,and the segment with taut-slack phenomenon is continuous near the anchorage.The slack of the mooring line starts from the anchor and passes from bottom to top,it passes to a certain position after a certain period of time.However,the change from slack to taut of mooring line starts from a position and is transmitted from top to bottom.After a certain period of time,it will be transmitted to the anchorage.(3)The effects of buoy on the characteristic of taut-slack,the mechanism of taut-slack alternation,the impact amplification coefficient,the maximum dynamic tension and its location,the length of submarine chain and the mooring line motion characteristics are analyzed.The results show that when the platform moves violently under extreme sea conditions,if the additional buoy is too large,it may cause more harmful taut-slack phenomenon at the end segment near the platform,and change the alternate process of taut-slack phenomenon.The addition of appropriate size buoy can reduce the possibility of occurring taut-slack phenomenon of mooring line,reduce the impact amplification coefficient,and even avoid the occurrence of taut-slack phenomenon.In addition,the additional buoy can reduce the maximum dynamic tension of mooring line and change its position.The additional buoy is also helpful to ensure the safe length of the laying chain,and has different effects on the motion of different nodes of the mooring line.(4)Based on the taut-slack characteristics of the mooring line and the maximum value of the impact amplification factor,the optimal installation position of several different parameter buoys was studied.The results show that the suitable installation locations of different size buoys are not consistent.In general,the larger the buoy is,the closer the optimal installation location is to the endpoint of the mooring line.When the buoy is not installed at the right position,it may cause the relaxation and tension phenomenon in the unit segment near the platform end under extreme sea conditions,which will adversely affect the safety of the mooring line and the stability of the platform.To some extent,the research in this paper determines the feasibility of expanding the application of catenary mooring system in deep-water and ultra-deep-water by means of additional buoy,which can provide certain guiding significance for the selection of buoy size and installation location in the mooring system.